Morphologic (light and EM) as well as cell kinetic studies have demonstrated the importance of the periodontal ligament (PDL) fibroblast in the maintenance of a healthy periodontium. Work of various investigators suggest that this population of cells function in the remodeling and turnover of the PDL, provides the cells required for cementogenesis, osteogenesis, and osteoclasis for maintenance of the functional integrity of the cementum and alveolar bone, and is involved in tooth eruption. Such varied functions indicate that this is a mixed cell population and that certain cellular elements within the pool might respond in a specific manner to different stimuli. The investigation is aimed at answering the questions: Under normal conditions, can the PDL fibroblast population be separated into functional subpopulations on the basis of the ultrastructural and EM cytochemical characteristics? If under normal conditions the cell population appears homogeneous, what specific ultrastructural/EM cytochemical changes appear in cells specifically stimulated to differentiate into either osteoblasts or osteoclasts? Also, what is the time response of these changes? In addition, the activity of the fibroblasts which do not respond to osteogenic stimuli and are involved in PDL repair will also be studied ultrastructurally and cytochemically. The functional activities of PDL fibroblasts will be studied and compared at the ultrastructural and EM cytochemical levels in normal rats and in rats in which either bone formation or bone resorption has been stimulated. Bone formation will be induced by mechanical movement of the teeth and bone resorption will be induced by administration of parathyroid hormone. Evidence for the existence of subpopulations of osteogenic cells (pre-osteoblasts and pre-osteoclasts) based upon organellar composition of the cells and the existence of specific enzyme activities will be sought. Since the model to be employed allows the specific stimulation of either osteoblastic or osteoclastic cytodifferentiation, the sequence of the ultrastructural/EM cytochemical events involved in each pathway can be investigated individually and compared.